Electrical resistance element and method of making the same



O. F. BOYKIN Sept. 6, 1966 ELECTRICAL RESISTANCE ELEMENT AND METHOD OF MAKING THE SAME Filed Sept. 20, 1962 IO-RESISTANCE ELEMENT [2-UNIFORMLY DISPERSED NOBLE METAL PARTICLES FIGURE- l0 :l- SUBSTRATE IO-RESlSTANCE I l-suasmma FIGURE- I OTIS F. BOYKIN R O T N E V W m m R N U T U 0 A o E T E M M w M E S 0% A M TL 65 T E s RT I- T L U M X B T. E m G EOAG Ms UC F T M F S M A m W0 m SR R C R N6 m0 T. l 0 E OR SM N F LC 0 L A .L IA G UH M 8 F MES T S M M fi U G m u m mw m m mm U0 MO m Lm HN PT ICM l O oal D-N m o VAN o MCAB A0 FDC OI CS FIGURE-2 BY ATTORNEY 3,271,193 ELECTRHCAL RESISTANQE ELEMENT AND METHUD 9F MAKING THE SAME Otis F. Boyltin, Chicago, lill., assignor to CTS Corporation, Elkhart, had, a corporation of lndiana Filed Sept. 20, 1962, Ser. No. 225,134 Qlaims. (Cl. 117-227) This invention relates to electrical resistance elements generally and in particular to an electrical resistance element which comprises a thin film of glass with particles of conductive metal dispersed therein supported by a nonconductive high temperature resistance base and the method of making the same.

The resistive elements of the type to which this invention relates are commonly called cermet resistive elements. The name of course implies a mixture of ceramic and metallic materials. The elements are formed from a mixture of finely divided glass powder and either finely ground metal particles or metal resinates. This mixture is placed on .a nonconductive substrate and fired to decompose the resinate if it is used and to fuse the glass powder into a solid glass film containing the metal particles.

Since the glass particles are relatively large and are of varying sizes, the distribution of the metal throughout the film is not uniform. The film gives the appearance of being homogeneous, but under electron microscopic analysis, the metal particles are found to be randomly deposited and of varying size. In fact, some metal particles appear to be 10 to 20 times the size of others indicating that the metal particles tend to agglomerate during the firing process as a result of their uneven dispersion in the tired composition. This nonuniform distribution of the metal particles in the film causes the contact resistance to vary as the concentration of metal particles in or near the surface of the film varies.

Variations in contact resistance produces noise. To keep the noise to a minimum the contact pressure is increased in an effort to reduce the contact resistance. This in turn greatly reduces the rotational life of the unit by increasing the friction between the contact and the element.

The contact resistance of these cermet" resistive elements would be greatly improved if the size of the metal particles and their distribution throughout the glass were more uniform. Therefore it is a principal object of this invention to provide a cermet element in which the electrically conductive metal particles are dispersed uniformly throughout the glass.

It is an additional object of this invention to provide a method of producing a cermet resistive element in which all the ingredients which make up the element are mixed as liquids which are completely miscible so that a homogeneous product results when the mixture is fired.

It is an additional object of this invention to provide a method of manufacturing a resistive element in which the glass is formed concomitantly with the reduction of the metal containing resinate during the firing operation to thereby produce a completely homogeneous mixture of electrically conductive metal particles and glass deposited on a nonconductive base.

It is a further object of this invention and an important feature thereof to provide a method of manufacturing a resistive element which does not necessitate the use of a ground glass frit.

Briefly, the invention comprises the use of a mixture of resinates containing glass producing ingredients instead of the prepared glass frit used in manufacturing the prior art resistive elements.

As pointed out above cermet resistive elements were 3,Z7l,l% Patented Sept. 6, 1966 heretofore made 'by combining the metal used to conduct the electricity with a ground frit of the desired glass.

The metal was added to the glass frit either as finely ground particles or as metal resinate which decomposed during the firing process leaving the metal dispersed in the glass film. The ingredients were thoroughly mixed mechanically, screened on a substrate and fired.

Regardless of how well these ingredients were mixed together, the glass frit would always have .a finite particle size, which when fired, would serve to space the metal particles at least as far apart as the diameter of the particles. As a result, the distribution of metal throughout the film depended on both the particle size of the glass and the thoroughness 'with which the ingredients were mixed. Also affecting the distribution was the fact that the particle size varied considerably throughout any given mixture.

This was a haphazard system and not only contributed to the contact resistance problem described above but also increased the production costs of the elements since the characteristics of elements manufactured from the same mixture were dilferent and highly unpredictable.

By using a mixture of resinates to replace the glass frit and a resinate containing the desired metal, all of which are liquid and therefore completely miscible in each other, a homogeneous mixture of the basic ingredients can be obtained which in turn produces a homogeneous resistive element.

For a better understanding of the present invention reference may be had to the accompanying drawing wherein the same reference numerals have been applied to like parts and wherein:

FIGURE 1 is a fragmentary isometric view of an electrical resistance element made in accord with the present invention;

FIGURE 1a is a grossly enlarged cross section of a portion of the resistance element of FIGURE 1 showing the uniformly dispersed noble metal particles in a glass matrix; and

FIGURE 2 is a diagrammatical representation of the invention.

Referring now to the drawings, there is illustrated an electrical resistance element generally designated at 10 comprising a substrate 11 of dielectric material such as alumina or steatite. Deposited onto a surface of the substrate 11 are uniformly dispersed noble metal particles 12, i.e., gold, silver, platinum, palladium, rhodium, iridium, osmium, and ruthenium in a glass matrix 13.

By way of examples, a resistive element 10 according to this invention may be made as follows:

Example I The following ingredients are tho-roughly mixed together.

Silver resinate (.2 gm. Ag/cc.) cc 5.0 Palladium resinate (.09 gm. Pd/cc.) cc 11.1 Rhodium resinate (.05 gm. Rh/cc.) cc 0.4 Lead resinate (.278 gm. Pb/cc.) cc 4.0 Boron resinate (.038 gm. B/cc.) cc 4.0 Screening agent gms 25.0

The percentage of metal shown for each resinate is percent by weight which will be true for all percentages shown throughout this specification unless otherwise indicated.

All of the above listed ingredients are all liquids with the exception of the screening agent which may be an emulsoid. Since it is added after the other ingredients are mixed together and is driven off first during the firing operation it has no effect on the homogeneity of the mixture of the liquid ingredients. They are completely miscible so a homogeneous mixture can always be obtained with a minimum of mixing. The screening agent can be one of any of those commercially available such as ethyl cellulose.

Next, the mixture is screened on a ceramic substrate 11 which will not soften appreciably during the firing process. One such ceramic is alumina; another is steatite either of which is suitable.

The substrate with the mixture screened thereon is fired at a temperature of about 800 C. for approximately 25 minutes. At this temperature all volatile liquids are driven olf; the resinates are decomposed with the hydrocarbon being burned off leaving the metal. The lead and boron will oxidize and combine to form a lead borate glass throughout which the silver, palladium, and rhodium are evenly distributed. After firing, the film is made up as follows:

Percent by weight Silver 30.5

Palladium 30.43

Rhodium .61

Lead 33.83

Boron 4.63

The film produced by this combination has a resistance per square of approximately ten ohms.

Example 11 The following liquid resinates and screening agent are thoroughly mixed together in the proportions shown and screened on a ceramic substrate.

Ruthenium resinate (.04 gm. Ru/cc.) cc 2.0 Lead resinate (.278 gm. Pb/cc.) cc 1.0 Boron resinate (.032 gm. B/cc.) cc 2.9 Silicon resinate (.15 gm. Si/cc.) cc 0.6 Screening agent gms 40.0

Ruthenium 14.35

Lead 49.80

Boron 19.70

Silicon 16.15

Example 111 The following ingredients are thoroughly mixed together:

Bismuth resinate (0.05 gm./cc.) cc 3.8 Lead resinate (0.3 gm./cc.) cc 4.1 Silicon resinate (.15 gm./cc.) cc 1.25 Manganese resinate (.135 gm./cc.) cc 0.2 Boron resinate (.05 gm./cc.) cc 4.75 Ruthenium resinate (.04 gm./cc.) cc 2.5 Screening medium gms 25 This mixture when screened onto a substrate and fired produces a lead boro-silicate glass which also contains bismuth and manganese. The ruthenium here as in Example II furnishes the electrically conductive metal which is disposed uniformly throughout the glass formed by the other metals.

In all of the examples given the percentage by weight of each metal is calculated without considering the fact that they are either completely or paritally oxidized during the firing process. To form glass, of course, it requires the oxides of the metals and since a glass is obtained it can be safely assumed that the metals which form the glass do oxidize after they are separated from resinates.

Also since the invention eliminates the glass frit, which possibly tended to shield the noble metal during the firing operation and retard its oxidation and due to the extreme thinness of this film being in the range of 5 to 20 microns, it is now believed that certain noble metals such as ruthenium, rhodium, and palladium do oxidize when fired. The amount of oxidation is not known, however, with some noble metals it could be This, of course, partially accounts for the change in electrical properties which occurs with changes in the firing time.

The resistance of the element after only one layer of the mixture is fired is generally too high. In fact, in some occasions it is non-conductive. To lower the resistance additional layers are fired. Each layer lowers the resistance until the desired value is obtained. It is usually desirable to have from two to five layers so that the element will have some thickness.

The resistance desired also determines what metals are used to form the element and their percentage of concentration. In Example I, silver and palladium were the conducting metals and they constituted slightly over 60% by weight of the total meterial in the film. The resistance of this element would naturally be low with such a high concentration of good conductors.

In Examples II and III, ruthenium was the conducting metal and the percentage of metal to glass was reduced so the resistance of the element was higher. In Example I, eight to twelve ohms per square was obtained after firing four layers while in Example II and III, four layers were fired to obtain 8000 to 12000 ohms per square and 2000 to 5000 ohms per square respectively.

These resistive elements are very smooth since they are obtained from liquid ingredients. For the same reason, they are completely homogeneous. When used in a variable resistor the contact pressure can be reduced due to their uniformity. The reduction in contact pressure combines with the smoothness of the element to greatly lengthen both the element and contact life of the control. When used as a fixed resistor, the uniform distribution of the metal throughout the glass tends to reduce the voltage coefficient. This feature also increases the reproducibility of the elements during the manufacturing process.

While there has been illustrated and described What is at present considered to be a preferred embodiment of the present invention and -a method of' making the same and a single modification thereof, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and the modifications which fall within the true spirit and scope of the present invention.

The invention claimed is:

ll. A method of manufacturing a resistive element comprising the steps of admixing a composition of a thermally decomposable organic metal compound containing at least one of the noble metals in solution and thermally decomposable glass-forming organic metal compounds in solution; applying a film of the composition onto the surface of a high-temperature-resistant, electrically nonconductive base; firing the mixture and the base at from 750- 900 C. to thermally decompose the organic metal compounds; and cooling the film and the base to solidify the 2. The resistive element as produced in accord with the process of claim 1.

3. A method of making a resistive element comprising the steps of preparing a completely homogeneous composition of organic metal compounds in solution having at least one noble metal and several glass-forming compounds attached to the organic compounds; applying a film of the composition to a high-temperature-resistant, electrically nonconductive base; firing the mixture at from 750900 C. for from 5-30 minutes to reduce the organic metal compounds and to combine the glass-forming compounds in situ to form a glass; and cooling the film and the base to solidify the film.

4. A method of making a resistance element comprising the steps of: admixing a composition of a thermally decomposable organic metal compound containing at least one of the noble metals selected from the group consisting of ruthenium, rhodium, iridium and palladium in solution and thermally decomposable glass-forming organic metal compounds in solution; applying a film 'of the composition onto the surface of a high-temperature-resistant, electrically nonconductive base; fin'ng the film and the base at from 750-900 C. to thermally decompose the organic metal compounds and simultaneously forming in situ a glass and an oxide of the noble metal uniformly dispersed in the glass; cooling the film and the base to solidify the film.

5. The resistive element as produced in accord with the process of claim 4.

1,977,625 10/1934 Ernst 117-123 2,440,691 5/ 1948 Iira 117-227 2,490,399 12/ 1 949 Ballard 106-1 2,551,712 5/1951 Soby 117-123 2,733,167 1/1956 Stooky 117-123 2,786,925 3/1957 Kahan 117-227 2,842,457 7/1958 Morgan et a1 106-1 2,855,493 10/ 1958 Tierman 117-227 2,924,540 2/ 1960 DAndrea 252-514 2,950,996 8/1960 Place et al. 117-227 3,052,573 9/1962 Dumesnil 252-514 3,149,002 9/1964 Place et al 117-227 FOREIGN PATENTS 577,384 6/1959 Canada.

699,019 11/ 1940 Germany.

625,198 6/ 1949 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

W. L. JARVIS, Assistant Examiner. 

1. A METHOD OF MANUFACTURING A RESISTIVE ELEMENT COMPRISING THE STEPS OF ADMIXING A COMPOSITION OF A THERMALLY DECOMPOSABLE ORGANIC METAL COMPOUND CONTAINING AT LEAST ONE OF THE NOBLE METALS IN SOLUTION AND THERMALLY DECOMPOSABLE GLASS-FORMING ORGANIC METAL COMPOUNDS IN SOLUTION; APPLYING A FILM OF THE COMPOSITION ONTO THE SURFACE OF A HIGH-TEMPERATURE-RESISTANT, ELECTRICALLY NONCONDUCTIVE BASE; FIRING THE MIXTURE AND THE BASE AT FROM 750900*C. TO THERMALLY DECOMPOSE THE ORGANIC METAL COMPOUNDS; AND COOLING THE FILM AND THE BASE TO SOLIDIFY THE FILM. 